Accelerometers measure the force of acceleration by the displacement of a mass in a spring-mass system. Optical accelerometers apply the displacement of the mass to modulate the intensity of a light beam, which in turn is detected by a photodetector, providing a corresponding electrical signal. Typically, the mass comprises a metal weight and the spring comprises an elastomer, metal or other material which provides a restoring force; the spring and mass together form a resonant system, whose resonant frequency defines the upper limit of linear response to the accelerometer. Below the resonance, the amplitude of the displacement depends only on the received acceleration and the natural resonant frequency of the resonant system. As the resonant frequency is increased, the displacement is reduced according to the inverse square of the resonant frequency. Specifically, in a linear system, the displacement is given by the formula EQU x=a/W.sup.2.sub.res
where x is the displacement, a is the acceleration and W.sub.res is the resonant frequency. Mechanical systems operating in other coordinate systems have equivalent equations, generally known in the art.
The upper frequency limit is important in particular applications such as seismic exploration, which requires a response up to about 500 Hz. It is therefore necessary to define the natural resonant frequency of the accelerometer resonant system to be 500 Hz or greater. Substituting a W.sub.res of 500 in the above equation, in order to detect accelerations of 10.sup.-7 g (g=acceleration of gravity) typically found in seismic applications, the resulting motion of the mass in the resonant system is only 10.sup.-13 meters.
Previous systems have attempted to increase the accelerometer sensitivity by incorporating increasingly precise optical systems to measure the resulting mechanical displacement. This improvement has typically concentrated on improving lenses or optical gratings used to reduce the optical spot size, and therefore increase sensitivity. In one approach, the light is focused to a sharp point, wherein the small motions of the mechanical system provide a large percentage variation in the intensity of the focused light. However, even with the best optics, there is a practical limit to the smallest acceleration, as represented by mechanical displacement, that can be detected. For instance, in the case of optical accelerometers with diffraction limited optics, the minimum detectable acceleration at low frequencies is not as good as presently available moving-coil geophones.
In accelerometers with precision optical systems, the optical elements must be critically aligned during manufacture. Moreover, accelerometers having highly sensitive optical systems are also generally sensitive to the operating conditions such as temperature, which may need to be carefully controlled.